In the field of radio communication systems it is a well known problem that the power amplifier of broadcast transmission equipment, such as the wireless base station 1 having antenna 2 illustrated in FIG. 1, operates in a non-linear fashion when the power amplifier is operated near its peak output. As a result the power amplifier introduces significant signal distortion, which can appear in various forms.
If more than one signal is input into the power amplifier or power amplification stage, its non-linear characteristics can cause an undesirable multiplicative interaction of the signals being amplified, and the amplifier's output can contain intermodulation products. These intermodulation products cause interference and crosstalk over the amplifier's operational frequency range, which interference and crosstalk can exceed regulatory broadcast transmission standards.
In addition to signal distortion taking the form of a degradation of the spectral components of the signal being amplified, it can also take the form of spurious spectral output outside of the bandwidth of the signal being amplified.
By reducing the amplifier output, one can make the amplifier operate substantially in its linear region. However, this also tends to diminish substantially the power conversion efficiency of the amplifier, so that a larger, more expensive amplifier would have to be used to support transmitter output power at a given level.
FIG. 2 illustrates a simplified block diagram of a typical base station transmitter operating with quadrature amplitude modulation (QAM). An in-phase component (I) and a quadrature component (Q) of baseband signal 6 are combined with a local oscillator 10 signal in modulator 8 and the resulting up-converted radio frequency (RF) signal is applied to the high power amplifier (HPA) 12. HPA 12 amplifies the up-converted RF signal for transmission by antenna 2.
A broadcast system employing QAM requires the transmitter to vary both the phase and amplitude of the transmitted signal. A power amplifier that performs non-linearly as it approaches its peak output generally has significant difficulty in implementing the QAM scheme, so that as a result spurious emissions are often transmitted out of an assigned RF channel, contrary to required industry standards and governmental regulations.
Besides the clearly sub-optimal solution of reducing the HPA output power level to achieve linear power output, it is known in the art to use various linearization schemes.
For example, FIG. 3 illustrates a block diagram of a high power amplifier (HPA) employing a feed forward linearization technique. The input to basic power module (BPM) 15 and the output of BPM 15 are both fed into a comparison circuit 16, which generates a difference output to error amplifier 20. The output of error amplifier 20 is applied to summing circuit 18, to which is also applied the BPM output. The BPM output is thus modified in response to differences in the content of the signal going into the BPM and that of the signal coming out of the BPM.
FIG. 4 illustrates a block diagram of a high power amplifier (HPA) employing a feedback linearization technique. The input to BPM 15 and the output of BPM 15 are both fed into a multiplier circuit 26, which generates an output to an analog processing circuit 28. The output of analog processing circuit 28 is applied to multiplier circuit 24, to which is also applied the signal which is to be amplified by BPM 15. The output of multiplier circuit 24 is input into BPM 15. Accordingly, the output of BPM 15 is modified in response to differences in the content of the signal going into BPM 15 and that of the signal coming out of BPM 15.
The techniques illustrated in FIGS. 3 and 4 require very expensive equipment to implement, in that among other things they require high performance analog circuitry that must be properly adjusted by skilled labor and which may subsequently go out of adjustment.
It is well known to reduce intermodulation distortion in power amplifiers by predistorting the signal to be amplified in order to cancel out the distortion that is produced by the amplifier. One type of predistortion employed in the art utilizes predistortion linearization, which will now be discussed briefly.
FIG. 5 illustrates a block diagram of a high power amplifier (HPA) employing a predistortion linearization technique. The signal to be amplified by the HPA is fed into a module 30 which functions according to an inverse model of the signal distortion characteristics of BPM 15. The signal to be amplified by the BPM 15 is also fed into a digital processing circuit 32 along with the output of the BPM 15. The output of the digital processing circuit 32 is coupled to module 30, whose output is coupled to the BPM 15. As a result, the output of BPM 15 is modified in response to differences in the content of the signal going into the BPM 15 and that of the signal coming out of the BPM 15.
Another type of predistortion employed in the art utilizes instantaneous predistortion linearization without memory, which will now be discussed with reference to FIG. 6. FIG. 6 illustrates a block diagram of a high power amplifier (HPA) employing an instantaneous predistortion linearization technique without memory. The signal to be amplified by the HPA is fed into an inverse distortion or predistortion module 40 which performs inverse distortion or predistortion of the signal in an attempt to cancel out the non-linear signal transfer characteristics of BPM 15. The signal to be amplified by the BPM 15 is also fed into a digital processing circuit 42 along with the output of the BPM 15. The digital processing circuit 42 generates model parameters in the form of a description 44 of an amplitude function based on adaptive parameters and a description 46 of a phase function based on adaptive parameters. The outputs of blocks 44 and 46 are coupled to module 40, whose output is coupled to the BPM 15. As a result, the output of BPM 15 is modified in response to differences in the content of the signal going into the BPM 15 and that of the signal coming out of the BPM 15.
The technique of using instantaneous predistortion linearization without memory suffers from an inability to track relatively quick changes in the ambient environment of the HPA, such as heating of the BPM due to relatively quick changes in the average power of the input signal, and to track changes in the bias conditions of the BPM due to relatively quick changes in the average power of the input signal.
The following patents were found in the course of a search for art relating to the inventions described herein:
U.S. Pat. No. 4,291,277 describes the use of an adaptive predistortion technique for linearizing a power amplifier. Digital data to be transmitted is used to access digital codes representing in-phase (I) and quadrature (Q) reference voltages from a memory. The digital codes are processed into a composite signal in the intermediate frequency range and input into the amplifier for transmission. Part of the amplifier output is fed back to a comparator network to be compared with the I and Q reference voltages, and any differences between the I and Q outputs of the amplifier and the respective reference voltages are summed with the I and Q components read out of the memory and are written into the memory as updated predistorted I and Q components.
U.S. Pat. No. 5,148,448 describes an adaptive predistortion circuit for predistorting input data before it goes into the amplifier. The predistortion circuit includes a memory which is continually updated by the results of a comparison of the input data and the amplifier output.
U.S. Pat. No. 5,870,668 describes an amplifier having distortion compensation and a base station for radio communication using such amplifier. The amplifier includes a coefficient-generating circuit for generating compensation coefficients to compensate for distortion characteristics of the amplifier. The coefficient-generating circuit is responsive to an error signal, generated in response to comparing the input signal with an output signal of the amplifier, based upon an adaptive algorithm.
The techniques for adaptive predistortion described in this section, including the above three patents, are generally complex and thus very expensive to implement in wireless base station equipment which is intended to be commercially competitive.
Therefore, there is a substantial need to provide an amplifier which operates in substantially linear fashion near its maximum output and which is relatively inexpensive to implement.
In addition, there is a substantial need for a method for operating an amplifier which operates in substantially linear fashion near its maximum output and which is relatively inexpensive to implement.
There is also a substantial need to provide a wireless base station comprising an amplifier which operates in substantially linear fashion near its maximum output and which is relatively inexpensive to implement.